Such pumps are already known. They serve to deliver hot measuring gases, the temperature of which is to be preserved as far as possible in the sampling condition. Condensation of constituents of the gas to be analyzed, falsification of the measurement result and so on are hence to be avoided. In order that the areas in such hot gas pumps coming into contact with the gas to be measured are kept at the proper temperature, usually predetermined by the sampling point of the gas to be measured, it is already known to incorporate a heating device in the pump head. The heating device is to prevent the temperature of the gas to be measured from falling in the area of the pump chamber or at least to reduce such temperature drop. Such a heating device or heat source is usually realized by an electric heating rod which is fitted, for instance, symmetrically in the pump head near the pump chamber and may be controllable with respect to its heat output.
However, such known measuring gas pumps, to be described in further detail below, still present some drawbacks. They are generally in the form of diaphragm pumps, the stroke being imparted to the diaphragm with the aid of a connecting rod. For this purpose the connecting rod is widened in a mushroom-like fashion at the upper end and engages under the working membrane which is located in position, in the direction of the pump chamber, by a fixing plate at the free end of the connecting rod. The crankcase for the connecting rod is situated beneath the mushroom of the connecting rod. This has the adverse side effect that the connecting rod case produces a cooling effect on the connecting rod mushroom and therefore also on the working diaphragm and directly on the gas to be measured.
If, as is often the case, an electric heating rod is accommodated in the pump head as a heat source, a temperature drop in the pump case ensues from there in the direction of the connecting rod drive. The then unavoidable cooling occurring at the connecting rod head is liable to affect the gas to be delivered and measured. The cooling of the connecting rod head and working diaphragm is partly promoted by air convection or the like occurring with an up-and-down travelling connecting rod in such a way that, as the connecting rod goes up, colder air from the crankcase is delivered into the region of the working diaphragm and, as the connecting rod goes down, heated air there is moved in the direction of the crankcase. As a secondary effect of the diaphragm drive, some regular air convection having a cooling effect on the working diaphragm is unintentionally created.